#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

#define MAXOP 100 
#define NUMBER '0'
#define MAXVAL 100
#define BUFSIZE 100

int getop(char s[]);
void push(double d);
double pop(void);
int getch(void);
void ungetch(int c);

int main(){
    int type;
    double op2;
    char s[MAXOP];

    while((type = getop(s)) != 'Q'){
        switch(type){
            case NUMBER:
                push(atof(s));
                break;
            case '+':
                push(pop() + pop());
                break;
            case '*':
                push(pop() * pop());
                break;
            case '-':
                op2 = pop();
                push(pop() - op2);
                break;
            case '/':
                op2 = pop();
                if(op2 != 0.0){
                    push(pop() / op2);
                }else{
                    printf("error: zero divisor\n");
                }
                break;
            case '\n':
                printf("\t%.8g\n", pop());
                break;
            default:
                printf("error: unknown command %s\n", s);
                break;
        }
    }
    return 0;
}

int sp = 0;
double val[MAXVAL];

void push(double f){
    if(sp < MAXVAL){
        val[sp++] = f;
    }else{
        printf("error: stack full,can't push %g\n",f);
    }
}

double pop(void){
    if(sp > 0){
        return val[--sp];
    }else{
        printf("error: stack empty\n");
        return 0.0;
    }
}

int getop(char s[]){
    int i, c;
    while((s[0] = c = getch()) == ' ' || c == '\t'){
        
    }
    s[1] = '\0';
    if(!isdigit(c) && c != '.'){
        return c;
    }
    i = 0;
    if(isdigit(c)){
        while(isdigit(s[++i] = c = getch())){
            
        }
    }
    if(c == '.'){
        while(isdigit(s[++i] = c = getch())){
            
        }
    }
    s[i] = '\0';
    if(c != 'Q'){
        ungetch(c);
    }
    return NUMBER;
}


char buf[BUFSIZE];
int bufp = 0;

int getch(void){
    return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c){
    if(bufp >= BUFSIZE){
        printf("ungetch: too many characters\n");
    }else{
        buf[bufp++] = c;
    }
}
